The present invention relates to a discharge lamp which is designed for dielectrically impeded discharges. For this purpose, the discharge lamp has a discharge vessel filled with a discharge medium, and an electrode arrangement with at least one anode and at least one cathode. Since the discharge lamp is designed for dielectrically impeded discharges, a dielectric layer is situated at least between the anode and the discharge medium. The anode and the cathode thereby define between them a discharge spacing in which dielectrically impeded discharges can be produced.
The terms anode and cathode are not to be understood in this case such that the discharge lamp would only be suitable for unipolar operation. It can also be designed for a bipolar power supply, in which case there is then no difference between the anode(s) and cathode(s), at least electrically. Consequently, in this application, what is said for one of the two electrode groups applies in the case of a bipolar power supply for both electrode groups.
The discharge lamps considered here have a large number promising fields of application. An important example is the backlighting of flat image systems, in particular LCDs (Liquid Crystal Displays).
A further point is the backlighting or lighting of signaling devices and signal lamps themselves. Reference is made regarding these two last points to the disclosure content, hereby referred to, of EP 0 926 705 A1. Reference is made, furthermore, to WO98/43277, also with regard to backlighting of flat display screens, and the disclosure content thereof is also referred to.
Since discharge lamps for dielectrically impeded discharges can be designed in the most varied sizes and geometries, and in the process achieve a relatively high efficiency on avoiding the typical disadvantages of classic discharge lamps with a mercury-containing filling, they are promising candidates for a large number of different technical fields of use.
Many technical attempts have been undertaken for the purpose in this case of maximizing parameters such as the light yield, the luminous flux, the luminance, the homogeneity of the luminance etc.
Particular reference is made regarding the prior art to patent abstracts of Japan 1996, No. 6, dated Jun. 28, 1996 and to the associated JP 08031387 A. This prior art describes a discharge lamp which is designed for dielectrically impeded discharges and in which a region with a small discharge spacing is provided in order to reduce the starting voltage. In one exemplary embodiment, this small discharge spacing is 2 mm. However, during operation discharges also burn in the whole discharge lamp in the region of larger discharge spacings.
The invention is based on the technical problem of improving a discharge lamp for dielectrically impeded discharges such that its possibilities of use are further expanded, and to specify a corresponding operating method for the discharge lamp.
Firstly, the invention proceeds from the finding that there is a range of applications for which it is important, in addition to, or instead of, the qualities required at the beginning, for it to be possible to operate the discharge lamp with a very low luminous flux. It was necessary for this purpose in the case of the invention to improve the properties of the lamp such that it permits the injecting of very low supply powers. This is possible according to the invention by virtue of the fact that the discharge spacing between the electrodes is selected to be particularly small. According to the invention, this discharge spacing between cathodes and anodes is 3 mm or less, preferably 2 mm, 1.5 mm, 1 mm, 0.8 mm or below and, with particular preference, 0.6 mm and below.
It is important in this case that electrode pairs with such a small discharge spacing need not occur exclusively in the discharge lamp. It is also perfectly possible to make use of larger discharge spacings in the same discharge lamp, because then it is possible, if appropriate, to operate the lamp, if required, only with the small discharge spacing according to the invention.
The substantial advantage of the short discharge spacings consists in that they permit particularly long dead times between the individual active-power pulses in the case of a pulsed power supply, without the production in this case of locally undesired high current densities.
Firstly, reference is made with regard to the operating method with the pulsed active-power injecting to WO 94/23442 and DE-P 43 11 197.1, whose disclosure content is hereby referred to.
In the case of this operating method, dead times during which no discharge burns in the discharge lamp occur between individual pulses in which the discharge lamp is supplied with active power. During the injecting pulses of active power, the discharge certainly need not burn continuously in this case; it is equally as little necessary for the discharge to be terminated directly after the end of the injecting of active power. In any case, specific dead times without discharges occur between the discharge ignitions during operation of the lamp.
If, now, the dead times between the discharges are greatly lengthened, this reduces the mean power thereby injected into the lamp, and therefore also the mean emitted optical power, at least as long as the amount of energy injected per pulse is not increased to compensate. Rather, it is preferred in the invention that the energy injected per active-power pulse remains substantially constantxe2x80x94including in the case of a power adjustment still to be treated belowxe2x80x94that is to say is not consciously changed. Of course, in this case it can change somewhat owing to the changing electroparameters and discharge parameters because of the lengthening of the dead time, but this does not invalidate the invention.
At the present state of knowledge, it is to be regarded as a purely empirical result that particularly long dead times are possible in the case of the small discharge spacings according to the invention. It was, rather, expected that arcs destroying the dielectric form, because there is virtually no longer any physical coupling owing to the excessively long dead times between the individual active-power pulses. In the case of the dead times xe2x80x9cof normal lengthxe2x80x9d, an individual discharge structure forms an ionization of the discharge medium which is removed after extinction of the discharge pulse. The next discharge pulse then ignites in a still somewhat preionized region of the discharge medium, thus also giving rise to the temporal and spatial homogeneity of the overall discharge picture which is the aim of the pulsed mode of operation.
If dead times now become too long, in the case of conventional discharge spacings this coupling no longer takes place between the individual discharge pulses, and so each discharge pulse is, as it were, comparable to a new ignition which initially exhibits an arc-shaped discharge. The arcs repeated with each pulse render completely impossible permanent operation of the lamp and efficient homogeneous production of light; the discharge lamp is, however, in general damaged and therefore destroyed earlier.
It was surprising, moreover, that also no substantial acoustic problems arose with the invention. In the case of xe2x80x9cconventionalxe2x80x9d discharge spacings, bothersome piping noises were found at excessively low frequencies, that is to say frequencies in the audible range, these noises being produced by coupling of the pulse frequency of the discharges to the discharge vessel via various mechanisms of no interest here. However, it is to be seen in the case of the invention that such problems virtually do not occur any more, probably owing to the small discharge spacings with a thereby reduced coupling, on the one hand, and probably owing to the in any case greatly reduced powers, on the other hand.
The invention therefore relates to an operating method in which the dead time between the active-power pulses can be set in order to set the lamp power, and this corresponds to a dimming method in the case of adjustability during operation of the lamp.
Above all, however, the invention addresses an operating method in which, as set forth above, use is made of particularly long dead times, in particular longer than the values already mentioned. This also includes operation of the discharge lamp at only this one low power, or the one long dead time.
It is also provided according to the invention for one or more further discharge spacings to be provided in a discharge lamp in addition to the small discharge spacing according to the invention. In this case, in particular, it is provided, specifically in combination with an auxiliary ignition function described further below, or independently thereof, to be able to operate these electrode groups with different discharge spacings separately. It is then possible during operation to operate various power stages with different electrode groups or different combinations of electrode groups, and thus to select optimum operating parameters in each case.
The disclosure content of DE 198 17 479 A1 is referred to in relation to the splitting of the electrode arrangement into separately operable groups.
In particular, electrode groups with a larger discharge spacing can be used for higher powers of the discharge lamp, because better efficiency is generally to be achieved given the larger discharge spacings. In any case, the small discharge spacings according to the invention are not really advantageous with regard to the efficiency of the production of light. However, this is generally of subordinate interest, because the aim is particularly low powers for which the absolute losses occurring from the worsened efficiency are low in any case.
A substantial problem in the efficiency of gas discharge lamps is, in particular, the heat budget which, however, does not play a critical role at low powers in the case of the worsening of the efficiency mentioned here, because, as mentioned, the losses are low in absolute terms.
If, then, a substantially lower power is to be adjustedxe2x80x94whether after switching on the discharge lamp anew, or for the purpose of dimming during operationxe2x80x94an electrode group (or a plurality of electrode groups) with the small discharge spacing according to the invention is used for this purpose below a specific power. If only discharges over the low discharge spacing are operated, substantial reductions in lamp power are possible in this case.
In order to ensure as continuous as possible a transition or a smooth dimming response, the discharge lamp is preferably designed such that the power ranges possible with the aid of the various discharge spacings overlap one another. In this case, upon xe2x80x9cswitching overxe2x80x9d from one discharge spacing to the other, jumps in efficiency, and thus discontinuous jumps in luminous flux given a continuous power characteristic, can certainly occur. By adapting the dimming response of a ballast with the aid of appropriate power jumps in order to compensate for jumps in efficiency when switching over between discharge spacings, it is, however, also possible to remove these small discontinuities if they are disturbing.
Finally, it is also possible during full-load operation of the discharge lamp to ignite discharges over all discharge spacings present, and thus to achieve a further power gain through the discharges over the small discharge spaces. This need not necessarily be linked to a loss in efficiency if, in accordance with the following explanation, an arrangement is selected in the case of which a certain auxiliary ignition function is present between different discharge paths. The so-called case losses can be reduced thereby.
With regard to the electrode arrangement of the discharge lamp, a particular configuration of the invention consists in that, in addition to an anode and a cathode (it being possible for further anodes and cathodes to be present), a further electrode is provided which is assigned to the anode and the cathode for the purpose of dielectrically impeded discharge, specifically to the cathode at the small discharge spacing according to the invention, and to the anode at a larger discharge spacing. Consequently, the additional electrode can act as an anode with regard to the small discharge spacing, and as a cathode with regard to the larger discharge spacing. This has the particular advantage that the xe2x80x9cdammingxe2x80x9d, caused by the particular mode of operation of the dielectrically impeded discharge, of electrons before the anode from the discharge over the short discharge spacing prepares, as it were, the discharge over the longer discharge spacing by virtue of the fact that the dammed electrons from the electrode then acting as cathode facilitate the ignition of this further discharge.
It is particularly preferred in this respect that the discharges over the smaller and the larger discharge spacing are not only operated together, that is to say simultaneously in the sense of macroscopic times, but that, moreover, a fixed phase relationship exists between the active-power pulses for the two discharges, said relationship being suitably selected with regard to the described ignition support function of the discharge over the smaller spacing for the discharge over the larger spacing.
It is helpful in this regard to be clear that the discharge over the small discharge spacing is very easy to ignite, because of this shortness of the discharge spacing, specifically also in the case of low powers. To that extent, it makes sense to support the comparatively difficult to ignite discharge over the larger discharge spacing by there already being an accumulation of electrons in the region of the cathode, that is to say on the dielectric and directly above the dielectric. (The electrode considered here must be covered by a dielectric in this embodiment, because it acts as anode, inter alia.)
It is to be stated in particular that the discharges over the larger discharge spacing can also be operated with substantially lengthened dead times by means of the auxiliary ignition function described. In connection chiefly with the above-described fixed phase relationship, this means in practice that the small discharge spacing is still switched on in the range of xe2x80x9cconventionalxe2x80x9d powers, the auxiliary ignition function permitting also discharges over the larger spacing to be dimmed down far below the conventionally achievable power range. In the case of very low powers, it is then possible in some circumstances to adjust a yet lower power by exclusive operation of the discharges with the small discharge spacing.
A further possibility which points in the same direction consists in replacing this xe2x80x9cdouble-function electrodexe2x80x9d by two electrodes. One of these electrodes is assigned as anode relative to the cathode provided at the small discharge spacing to the extent that the other of these electrodes is assigned as cathode in relation to the anode provided at the larger discharge spacing. When these two electrodes are sufficiently closely adjacent, an auxiliary ignition function in the already described sense is likewise possible.
In accordance with a further aspect, the already described measures according to the invention are supplemented by a configuration of the electrode arrangement in favor of dimmability already in the case of conventional discharge spacings. For this purpose, the electrode arrangement is configured inhomogeneously along a so-called control length such that there is a change in a burning voltage of the discharges within the control length. For the sake of clarity, reference may be made here to the preceding German patent application xe2x80x9cDimmbare Entladungslampe fur dielektrisch behinderte Entladungenxe2x80x9d [xe2x80x9cDimmable discharge lamp for dielectrically impeded dischargesxe2x80x9d] dated Sep. 29, 1998, file reference 19844720.5. The disclosure content of this application is, once again, incorporated by-reference.
A sinusoidal course at least of some of the electrodes is particularly preferred in this connection, the inhomogeneity being represented as a change in the discharge spacing, and thus in the burning voltage.
As already mentioned, the method according to the invention for power adjustment, or the dimming method, uses the dead time between individual active-power pulses of a pulsed power supply as parameter for influencing the power. Within the framework of this invention, two concrete variants are preferred for configuring a corresponding electronic ballast. These two variants are summarized in claims 17 and 18. For further details, reference is made, in turn, to prior applications, specifically to the applications entitled xe2x80x9cElektronisches Vorschaltgerat fxc3xcr Entladungslampe mit dielektrisch behinderten Entladungenxe2x80x9d (xe2x80x9cElectronic ballast for a discharge lamp with dielectrically impeded dischargesxe2x80x9d) 198 39 329.6 and 198 39 336.9, which come from the same applicant, as do all other cited applications. The disclosure content of these applications is also hereby referred to. The electronic ballasts described there using the forward converter principle, or using the flyback/forward converter principle, are clocked via a primary circuit switchxe2x80x94denoted there by TQxe2x80x94which is switched by a control devicexe2x80x94SE there. To this extent, the dead time can be influenced by suitable selection of the electric parameters of the ballasts and of the discharge lamp by appropriate intervention in the control logic of this control device. Thus, the value of the dead time can be influenced by externally influencing a reference variable of this control device for time definition. Details on this are clear to the person skilled in the art.
In the combination of the described operating method according to the invention and the described discharge lamps according to the invention, the invention relates to a lighting system with such a discharge lamp and an electronic ballast which is designed correspondingly and is not necessarily as claimed in claims 17 and 18.
As already mentioned at the beginning, display screens, signal lamps, lighting and backlighting of signaling devices etc. come into consideration as preferred application. By way of generalization, this field of application can be summarized by information displays of any type at all. Specifically, when information is being displayed, the readability of the information from the display device under different ambient conditions plays a very substantial role. This relates chiefly to the freedom from glare in the case of rather dark ambient conditions, and to the readability in the case of brighter surroundings or unwanted light. An adjustable power range of the discharge lamps which is as wide as possible is highly important for adaptation.
This relates, chiefly, to the field of traffic technology, for example lamps in the interior of vehicles. Reference is made here by way of supplement to the disclosure content of EP 0 926 705 A1 (already cited). As already stated, monitors and display screens also come into consideration. There is a need there for adjustment ranges for the luminous flux, typically 1:100, which cannot be implemented even approximately with discharge lamps without the invention (typically 1:5 to date). The field of office automation, for example lamps in scanners, also comes into consideration.